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Helpful Hints

Helpful Hint #1: Using Inexpensive LEDs as Optical Detectors

For some applications, an inexpensive, readily- available, LED can be used as an optical detector. Although not generally touted as optical detectors, most LEDs can be used as detectors without harming the device in any manner. In fact, most LEDs can perform double-duty in the same circuit without changing the LEDs physical or electrical connections.

The trick is to electrically bias the LED in the proper current-voltage (I-V) quadrant for operation as a detector and to detect an appropriate range of wavelengths.

The LED, as the name (Light Emitting Diode) implies, is electrically a diode and can be used as a detection device similar to a photodiode. LEDs are first cousins to photodiodes and have some similar optical and electrical properties.

Rule of thumb #1: LEDs will not respond as rapidly to optical rise and fall times and PN or PIN photodiodes.
A properly biased, good quality photodiode will have sub-nanosecond time response and be able to detect the rise and fall time of light pulses shorter than 1 nanosecond in duration, an LED used as a photodiode may only be able to detect 100 microsecond optical rise times and may not do well in detecting pulses shorter than a millisecond. However, if you are trying to detect if the room lights were turned on or if an outside door was opened during the day, this might be the ticket to an inexpensive, readily available optical detector.

Rule of thumb #2: LEDs will only detect light of wavelength shorter than the wavelength of light that the LED would emit if it was put in a circuit that forward biased the LED.
For example, a red LED will detect light emitted by a yellow LED and a yellow LED will detect light emitted by a green LED but a green LED will not detect light emitted by a red or yellow LED. All three LEDs will detect "white" light or light from a blue LED. White light contains a blue light component which can be detected by the green LED. Recall that visible light wavelengths can be listed from longest wavelength to shortest wavelength as Red, Orange, Yellow, Green, Blue, Indigo, Violet (remember the mnemonic "Roy G. Biv"). Violet is the shortest wavelength light with the most energetic photons and red has the longest wavelength light with the least energetic photons of all of the visible colors of light.

Rule of thumb #3:To use the LED as an optical detector, do not forward bias the LED into quadrant # 1 of the current-voltage (I-V). (Quadrant 1 is when the operating voltage and current are both positive.) Allow the LED to operate in the solar cell mode, quadrant #4 (operating voltage is positive, current is negative), or in the photodiode mode quadrant #3 (operating voltage is positive, current is negative).
Photodiodes are typically operated in quadrant 3 of their I-V characteristics. In this mode, a small reverse bias voltage is applied to the device and the incident light linearly increases the leakage current proportional to the intensity (actually the number of above bandgap photons) of light falling on the photodiode die. Although LEDs are not intended to experience large reverse bias voltages, most can be reverse biased by a few volts (3v to 7v) and operate in the photodiode mode. Make sure you limit the magnitude of the reverse current so that you do not damage the LED. The photodiode mode will give the best time response and the most linear sensitivity to light. You can expect sensitivities of a few tenths of a microamp of current for each microwatt of optical power directed at the LED (or photodiode die).

In the solar cell mode, no applied bias voltage is used. The solar cell (or LED in this case) generates its own current and voltage.

To make the LED act as a current generating device proportional to the magnitude of light incident on the LED die, use the "short circuit mode". External circuit conditions and the amount of light incident on the LED die determines the current-voltage operating point of the device. If you keep the generated voltage across the LED low by using circuit techniques to effectively put a short circuit across the LED, the generated current will be approximately linearly related to the amount of light incident on the LED. This becomes similar to the photodiode mode. The simplest approach to this circuit is to put a relatively small (know value) resistor across the two terminals of the LED and measure the small voltage generated by the current flowing through your known resistor. Experiment with the size of the resistor; the best value will depend on the amount of light you are detecting and the optical arrangement to capture this light onto your LED. Try 100 ohms, 1,000 ohms and 10,000 ohms. If you are using the NEEM-112 to detect this signal, use one of the +/- 2.5 volt channels for 16 bit sensitivity (approximately 75 microvolts resolution).

To make the LED act as a voltage generating device, operate in the open-circuit mode by connecting either no load resistor or as large a resistive load to the LED terminals as possible. In the no load resistor mode, the voltage measuring instrument will serve as the load. If the input impedance of the measuring instrument is high enough, the circuit conditions attached to the LED will appear as an open circuit condition. If you are using the NEEM-112 to detect this signal, use any of the voltage sensing channels including either of the 0 to 5 volt channel, either of the +/- 2.5 volt channels, or the +/- 10 volt input channel. In this voltage generating, open circuit mode, the LED will generate an open circuit voltage between 0.0 volts and approximately 1.6 volts - depending on the amount of light incident on the LED die and the color of the LED selected for this measurement. Remember, in the open circuit mode, the magnitude of the voltage is NOT linearly related to the amount of light incident on the LED die.



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